The present invention generally relates to electronic filters, and more particularly, to tunable filters.
Electrically tunable filters have many uses in microwave and radio frequency systems. Compared to mechanically and magnetically tunable filters, electronically tunable filters have the important advantage of fast tuning capability over wide band application. Because of this advantage, they can be used in the applications such as LMDS (local multipoint distribution service), PCS (personal communication system), frequency hopping, satellite communication, and radar systems.
One electronically tunable filter is the diode varactor-tuned filter. Since a diode varactor is basically a semiconductor diode, diode varactor-tuned filters can be used in monolithic microwave integrated circuits (MMIC) or microwave integrated circuits. The performance of varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range, and figure of merit, or Q factor at the specified frequency range. The Q factors for semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly.
Since the Q factor of semiconductor diode varactors is low at high frequencies (for example,  less than 20 at 20 GHz), the insertion loss of diode varactor-tuned filters is very high, especially at high frequencies ( greater than 5 GHz). Another problem associated with diode varactor-tuned filters is their low power handling capability. Since diode varactors are nonlinear devices, larger signals generate harmonics and subharmonics.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film dielectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film dielectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194.
Commonly owned U.S. patent application Ser. No. 09/419,219, filed Oct. 15, 1999, and titled xe2x80x9cVoltage Tunable Varactors And Tunable Devices Including Such Varactorsxe2x80x9d, discloses voltage tunable dielectric varactors that operate at room temperature and various devices that include such varactors, and is hereby incorporated by reference.
Combline filters, using resonant cavities, are attractive for use in electronic devices because of their merits such as smaller size, wider spurious free performance compared to the standard waveguide based cavity filters.
There is a need for tunable filters that can operate at radio and microwave frequencies with reduced intermodulation products and at temperatures above those necessary for superconduction.
Voltage-controlled tunable filters constructed in accordance with this invention include first and second cavity resonators, means for exchanging a signal between the first and second resonators, a first voltage tunable dielectric capacitor positioned within the first resonator, means for applying a control voltage to the first voltage tunable dielectric capacitor, a second voltage tunable dielectric capacitor positioned within the second resonator, means for applying a control voltage to the second voltage tunable dielectric capacitor, an input coupled to the first coaxial resonator, and an output coupled to the first coaxial resonator.
In a first embodiment of the invention, each of the first and second voltage tunable dielectric capacitors includes a first electrode, a tunable dielectric film positioned on the first electrode, and a second electrode positioned on a surface of the tunable dielectric film opposite the first electrode.
In another embodiment, each of the first and second voltage tunable dielectric capacitors includes a substrate, a tunable dielectric film positioned on the substrate, and an electrode positioned on a surface of the tunable dielectric film opposite the substrate. The electrode can be divided into first and second electrodes, separated to form a gap.
An insulating material can be included for insulating the first and second electrodes from the resonator. The tunable dielectric film can comprise barium strontium titanate or a composite of barium strontium titanate.
The voltage-controlled tunable filter can further comprise a first rod positioned in the first resonator, wherein the first voltage tunable dielectric capacitor is positioned at one end of the first rod, and a second rod positioned in the second resonator, wherein the second voltage tunable dielectric capacitor is positioned at one end of the second rod. Each of the rods in the coaxial resonators can be serially connected with one of the voltage tunable dielectric capacitors, and a second end of each of the rods can be connected to ground.